[BigEdLB]

Isn’t Mt. Rushmore sandstone?

[Smokin' Joe]

Nope. PreCambrian Granite. There is a batholith under the Black Hills. If you are into minerals, that particular granite has abundant black tourmaline (var. schorl) in it, especially in the vicinity of the visitors' center and the faces. (But you can't pick it up. It is a National Memorial.)

basic geologic map of the Black Hills and for the more serious rockhound, Mount Rushmore National Memorial Geologic Resource Evaluation Report